This invention relates to magnetite useful as pigment. In particular it relates to synthetic rhombohedral magnetite which is useful as a black pigment and which is also capable of being calcined in the presence of oxygen into an alpha-ferric oxide red pigment. This magnetite is prepared from waste pickle liquor by the addition of carbonate, preferably in the form of limestone or soda ash.
Numerous patents have issued in the past for processes of making iron oxide from pickle liquor. The following U.S. and foreign patents are representative of the technology as it exists today: U.S. Pat. Nos. 1,269,442; 1,824,936; 3,261,665; 3,434,797; 3,617,560; 3,617,562; 3,927,173; 4,090,888; 4,107,267; and U.K. Pat. No. 1,218,601. All of these patents call for the treating of waste pickle liquor with some form of base to produce an iron oxide. In most cases the iron oxide produced is a black oxide which can be used as a pigment and in some cases this black oxide is further calcined to produce a brown or red pigment.
It is the present invention that for the first time produces from waste pickle liquor and carbonate a synthetic rhombohedral magnetite which is superior as to tinting strength and unique as to particle size as evidenced by the high surface areas observed. Furthermore, this novel magnetite may be calcined in the presence of oxygen to a red pigment of very acceptable color characteristics and of a very low cost.